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March 7, 2006

Q&A: The Hazards of Fine Particulate Matter

Francesca Dominici, PhD

Researchers from the Johns Hopkins Bloomberg School of Public Health today published a study in the Journal of the American Medical Association relating hospital admissions with fine particulate matter levels. In a study of 204 U.S. urban counties, for every 10 micrograms per cubic meter of air (µg/m3) increase in particulate matter, the researchers calculated 11,000 additional cardiovascular and respiratory disease hospitalizations among the Medicare participants who participated in the study.

Kenna Lowe, with the Bloomberg School’s Office of Communications and Public Affairs, spoke to Francesca Dominici, PhD, associate professor in the Department of Biostatistics and lead author of the study, to learn more.

Question: With regard to your study results, what should people be concerned with?

Answer: Our study provides new and strong evidence that fine particulate matter levels still adversely impact daily hospital admission rates for cardiovascular and respiratory diseases in the United States. Among the 204 counties included in our study, in 2002, there were 1.4 million hospitalizations for cardiovascular and respiratory diseases. For every 10 µg/m3 increase in particulate matter, we calculated 11,000 additional cardiovascular and respiratory disease hospitalizations. This means that a reduction in fine particulate matter levels should provide health benefits in terms of lower hospital admissions for the elderly.

This was the largest study ever conducted to assess the increased risk of hospital admissions for cardiovascular and respiratory diseases associated to exposure to fine particles at a national and regional scale. Our study included 11.5 million Medicare enrollees during 1999-2000.

Question: Let’s back up a bit. What exactly is fine particulate matter?

Answer: Particulate matter is an airborne mixture of solid particles and liquid droplets. These solid particles come in numerous shapes and sizes and may be composed of different chemical components. Fine particles measure 2.5 micrometers or less in size. They are approximately 1/30th the diameter of a human hair and can penetrate deep into the body’s respiratory system. More information can be found at

Question: Is there a standard limit for particulate matter concentration in the United States?

Answer: The United States Environmental Protection Agency (EPA) regulates particulate matter by setting a standard and allowing each state to determine how to achieve that standard. The recently proposed National Ambient Air Quality Standard (NAAQS) for fine particulate matter is 35 µg/m3 for a 24-hour average; the annual average should not be more than 15 µg/m3. Many regions, including parts of the eastern United States and California, exceed these standards. More information can be found at

Question: What are some of the sources of fine particulate matter?

Answer: Airborne particles come from a variety of sources, including coal-burning power plants, factories, automobiles, agricultural activities, stone crushing and the burning of wood. Other particles may be formed in the air when sunlight and water vapor react with gases emitted from burning fuels.

Question: Now, let’s get back to your study results. How often was there a 10 µg/m3 increase in fine particulate matter? Also, how often did the daily level of fine particulate matter exceed the proposed NAAQS?

Answer: It is important to explain first that, on bad days, fine particulate matter levels are between 60 to 100 µg/m3. The average daily measurement of fine particulate matter during our study was 13.4 µg/m3.

Among the 204 counties included in our study, we found that, on average, approximately 20 percent of the time, the daily fine particulate matter level exceeded the annual average by more than 10 µg/m3 or approximately once every five days. We also found that, on average, approximately three percent of the time, the daily fine particulate matter level exceeded the proposed NAAQS standard—meaning that our estimated risks are associated with levels of fine particles that were below the proposed standard of 35 µg/m3 for a 24-hour period.

Question: Why did you find higher risks for cardiovascular hospital admissions in the East part of the United States as compared to the West?

Answer: The geographical variability of the estimated risks across the country may reflect the complexity of airborne particulate matter, which is made up of a rich mixture of primary and secondary particles. Combustion sources—vehicles, power generation and industry—are major contributors to urban particulate matter and vary by geographic location. Identifying factors that might explain east/west differences in risks and particulate matter components with higher toxicity is a very complex research question that we are just starting to explore.

Question: The study results showed a 1.28 percent increase in the risk for admission for heart failure as it related to increased fine particulate matter levels. How much of a concern is that really?

Answer: What we found was that a 10 µg/m3 increase in particulate matter was associated with a 1.28 percent increase in the risk of admission for heart failure. To further answer this question, I’ll use Cook County, which includes Chicago, as an example. It has average fine particulate matter levels of 16 µg/m3. For each 100 admissions, Cook County had approximately one extra hospital admission for heart failure associated with each daily 10 µg/m3 increase in the level of fine particulate matter. However, this percent increase is tied to a particular increase of fine particulate matter. If a larger increase in fine particulate matter pollution were to occur, we would have a larger increase in hospital admissions. The same goes for a decrease in particulate matter levels; if there was a smaller increase in pollution levels, there would be fewer additional hospitalizations.

Question: What else should the readers know about your study?

Answer: As you may know, scientific evidence is strengthened when the findings are replicated by different researchers. Some of my same colleagues who worked on this particular study are also promoting reproducibility of epidemiologic studies as a new minimum requirement for all researchers. This study is an example of a reproducible epidemiological study (Peng et al 2006). The dataset that we used to produce these study results—which includes county names and location, air pollution data, weather data, county-specific estimates of health risk and software developed to construct county-specific daily time series data—is available online at Other scientists can reproduce some of the study results, apply the study’s methodology to their own data or apply their methodology to our data.

More information on the Medicare Air Pollution Study 1999-2002 can be found at

News Release: Fine Particles Increase Hospital Admissions for Heart Failure and Cardiovascular Disease

Public Affairs media contacts for the Johns Hopkins Bloomberg School of Public Health: Kenna Lowe or Tim Parsons at 410-955-6878 or